Many days when I'm surfing ZAM, I get error messages along the lines of "Service Temporarily Unavailable" roundabouts 11PM EST.
I assume it's something simple like your server rebooting, but really, if that's it, you could totally push that to 5AM EST, when your primary audience of North Americans are fast asleep.
And if it's not something planned.... then maybe you should investigate it. I'll try to get an exact error message next time it happens.